Hi Bono,
My suggestion is don't begin your math prep from OG or any other study material (kaplan etc.).
I began my maths prep by taking a diagnostic test, so as to realise my strengths and weaknesses. Initially I was not comfortable with premutation- combination and number system problems. I took help of some elementary books on mathematics, and praticed many questions on these two topics.
I will advise you not to memorise anything other than formulaes of geometry, and if you have any problem remembering them just make a chart of all the formulaes and hang it on the wall and just give a look to it whenever u wish.
In my opinion if one knows trignometric ratios ( values of sine, cosine, and tangent from 0-90) and some basic formulas of coordinate geometry it will certainly help.
Moreover, I won't suggest anyone to use number fitting approach as suggested by kaplan. It is always better to solve the questions in conventional manner.
If you have anyother query or any problem in any topic, you can write to me.